The United States and Russia reached agreement tonight on a blueprint for Moscow's participation in a Kosovo peacekeeping force that prescribes an unprecedented degree of military cooperation between Moscow and the Western military alliance long considered its most mortal threat.

The deal will allow 3,600 Russian soldiers to patrol sectors of the southern Serbian province controlled by American, French and German troops. It also defuses a confrontation between NATO forces and some 200 Russian soldiers who have blocked access for the past week to Kosovo's main airport that was supposed to serve as NATO's command headquarters.

The breakthrough came after more than 30 hours of tense negotiations conducted over the past three days between Defense Secretary William Cohen and Russian Defense Minister Igor Sergeyev that sought to reconcile NATO's insistence on preserving its unified command structure with Russia's demand that its soldiers must not answer to alliance commanders.

At a signing ceremony at the Finnish presidential palace, Cohen and Sergeyev acknowledged the agreement raises the stakes in the Russian-NATO relationship because the two sides are now clearly embedded in a long-term strategic partnership to stabilize the southern Balkans, Europe's most volatile region.

Cohen said NATO managed to preserve its key demands that a NATO-led force in Kosovo must maintain unity of command and there must be no partition of the province by providing a geographic sector for the Russians. At the same time, he said Russia would keep its soldiers under its own authority by having Russian liaison officers installed at each layer of NATO's command structure.

"This agreement shows that the United States and Russia can work together on important security issues," Cohen said.

Sergeyev reaffirmed that Russia would maintain "complete political and military control" over forces in Kosovo even though its operations will be subject to the approval of national commanders in the U.S., French and German sectors. "We found solutions to important issues and we are satisfied with the results of our work," Sergeyev said.

The agreement is based on the Bosnian peacekeeping model, in which 1,300 Russian soldiers answer to a U.S. general who in turn responds to a Russian liaison officer at NATO headquarters in Brussels. But the Kosovo deal goes further by having the Russians deal with different allies and by inserting Moscow's liaison officers at each level of NATO's command hierarchy.

Senior U.S. officials said nearly 3,000 Russian troops would be equally distributed among the three allied sectors, with 750 others assigned to maintenance and fuel supply duties at the airport in Pristina, the capital of Kosovo. NATO personnel will handle all flight plans and air control matters.

About 200 Russian troops have occupied the airport since last Saturday, when they arrived in the early morning to cheering crowds of Kosovo Serbs following a day-long dash from Bosnia to beat NATO forces into the Serbian province.

Moscow will maintain authority over its own soldiers but coordinate their activities with respective national commanders in each sector, who in turn will answer to NATO's chief commander in Kosovo, British Lt. Gen. Mike Jackson. In effect, this system ensures that NATO will remain in charge, but Russia can claim to perpetuate its own chain of command.

"We got what we needed in that there will be no Russian sector, no partition and no alternative to a unified NATO command structure," said a senior administration official. "The Russians finally realized those were red lines that none of the allies would cross."

During the negotiations, the biggest obstacle was the persistent claim by Russian President Boris Yeltsin that Russia must be granted its own slice of territory in Kosovo separate from the five sectors to be run by the United States, Britain, France, Italy and Germany. But the Western allies rejected Moscow's demand, arguing that it would create a partition reminiscent of the Cold War era. Russia is an ally of Serbia, the dominant republic in the Yugoslav federation.

The negotiations were also complicated by the need to coordinate all arrangements with several capitals. "We were negotiation for NATO, not just the United States," said Pentagon spokesman Kenneth Bacon.

Secretary of State Madeleine K. Albright, who also attended the talks, stayed in frequent touch with European foreign ministers, particularly Hubert Vedrine of France and Joschka Fischer of Germany, whose governments demanded close consultation on all details about the nature of Russian military activities in their respective sectors.

At tonight's ceremony, Albright said the important achievement was that Russia and NATO troops working together in Kosovo would establish a force that is "everywhere strong, and everywhere respected" -- a clear reference to the Russian kinship with the Serbs and the sympathy felt by many of Kosovo's ethnic Albanians toward NATO.

In Cologne, where leaders from the Group of Seven major powers opened their annual summit today, President Clinton was informed late this afternoon by Cohen that the last conceptual hurdle had been cleared, U.S. officials said. But they remained cautious about public pronouncements, fearing a collapse of efforts to codify Russia's military operations in the allied sectors.

Russian Prime Minister Sergei Stepashin, who arrived in Cologne today to participate in economic discussions ahead of Yeltsin's planned appearance Sunday for further talks about Kosovo, said Moscow's role in the Kosovo force was imperative to ensure the security of Serb civilians from vengeful acts by the ethnic Albanian majority.

"There must be no double-standard approach toward the settlement of the situation when some [ethnic Albanian] refugees are coming back home while others have to flee their places. If there is a peace, it should be a peace for all," Stepashin said.

U.S. officials said the Russians have stepped up their warnings about the need for the NATO-led peacekeepers to uphold their promise to demilitarize ethnic Albanian guerrillas belonging to the Kosovo Liberation Army. Russian troops fear they could become targets for KLA attacks when they start to patrol in different sectors.

Albright today dispatched State Department spokesman James P. Rubin to Albania, from where he plans to proceed to the KLA's border strongholds to press the group's leader, Hashim Thaqi, to sign a demilitarization accord as quickly as possible.